This invention relates in general to data decoding systems and, more particularly, to a method and apparatus for recovering a clock signal and a data signal from a received encoded information signal.
Information signals such as digital signals can be encoded on carrier signals in several different ways. For example, a digital signal including a sequence of ones and zeros may be encoded on a carrier signal by using phase shift keying, frequency shift keying (FSK) and other encoding techniques prior to transmission. A significant characteristic of the carrier signal in an FSK system is the time period allocated for each digital data bit. FIG. 1 shows an example of a digital signal 10 in which each bit of the digital signal exhibits a bit duration, bit length or bit time of time T1.
One way of encoding digital signal 10 on a carrier signal for transmission is to use the data coding scheme of the Institute Of Electrical And Electronic Engineers (IEEE) standard 802.4 five megabit carrier band network. When digital signal 10 is encoded using this technique, a resultant encoded signal 20 (also designated V.sub.E) is generated as shown in FIG. 2. More specifically, the zeros (0's) of digital signal 10 are represented in encoded signal 20 by two full cycles of a 10 MHz signal designated HH (high frequency), while the ones (1's) of signal 10 are represented in encoded signal 20 by one full cycle of a 5 MHz signal designated LL (low frequency). IEEE standard 802.4 establishes a Medium Access Control (MAC) symbol time period which is equal to the time duration of one full cycle of a 5 MHz sinusoidal signal. Digital signals such as logical one's and zero's are defined by the frequency of the sinusoidal signal occuring during the defined time period.
Once the digital signals 10 are encoded on a carrier in a manner such as that described above, the encoded signals 20 are transmitted to a desired location via hardwire or radio transmission. Once the encoded signal 20 is received at the desired location, signal 20 must be decoded to restore the original digital information signal. One problem with decoding of such FSK signals of only one or two cycle duration is to synchronize a clock at the receiver with the received signal.